This invention relates to a composition for a joint compound for use in filling and coating the joints between adjacent panels of gypsum wallboard. More specifically, it relates to a composition for a lightweight joint compound of the setting or drying type that generates less airborne dust when the set or dried joint compound is sanded, and additionally provides for more uniform gloss retention upon painting.
In the construction of buildings, one of the most common elements is gypsum wallboard, often known as drywall, used in the construction of walls and/or ceilings. Walls made from gypsum wallboard are traditionally constructed by affixing the panels to wood studs or metal framing, and treating the joints between adjoining panels with a specially prepared adhesive called a joint compound. The seams between adjoining panels are coated with joint compound in such a way that a monolithic surface is created when finished. This process generally proceeds by placing a taping joint compound over the joint formed by the abutted edges of the wallboards, and embedding a liquid-permeable paper or fiberglass reinforcing tape within that compound. When dry (or set), a second coating referred to as a topping joint compound is applied over the joint, which may be subsequently lightly sanded upon drying. A third or finish coat is applied, allowed to dry, and lightly sanded to create the smooth monolithic surface that conceals any indication of where the drywall seams were. Another type of joint compound is an all-purpose grade that may be used for both embedding the joint tape and for applying the finish coats. A patterned or textured effect may be given to the finished wall and joint through the use of special application tools.
There are several categories of joint compounds. Drying type compounds cure through the evaporation of water, whereas setting type joint compounds chemically react with water during the curing process. Setting type joint compounds typically use calcium sulfate hemihydrate, also known as stucco or Plaster of Paris, as a base. When water is added to the setting type powder, it reacts with the calcium sulfate hemihydrate via a hydration reaction to form an interlocking matrix of calcium sulfate dihydrate crystals. The interlocking crystal matrix gives the compound increased strength. The benefit of a setting type joint compound over a drying type is an independence from having to wait for the joint compound to be completely dry prior to further finishing, as well as less shrinkage and cracking, and a higher overall strength of the finished joint. Drying type joint compounds have the advantage of ease of use, as they typically come in a ready mixed form, with water being added and mixed by the manufacturer. A third type of joint compound combines the setting action of a calcium sulfate hemihydrate based compound with the ease of use of a ready mixed compound. The properties of a ready mixed setting type joint compound are taught in U.S. Pat. No. 5,746,822, incorporated herein by reference.
Between coats of joint compound, and before the wall is finished, it is necessary to sand the joint compound to even the surface. While doing so, the dust generated is generally very fine and much of it tends to remain airborne, settling out only after long periods of time. This long residence time in the air allows it to travel long distances from the site of the sanding. During sanding of a home repair, for example, this fine dust is not confined to the area immediately adjacent to where the joint compound is being used, but is often found throughout the house, resulting in fine dust settling everywhere.
Liquid paraffin and oils are known to reduce dust in plaster-based compositions, such as those disclosed in PCT Publication WO/00/34200. However, these additives have a number of drawbacks. When such liquids are included in the applied composition, they tend to migrate toward the surface of the workpiece, giving an uneven distribution of dust reducing properties within the applied composition. The migration may cause the plaster to dry unevenly as well. Oil and liquid waxes also can cause loss of adhesion to the substrate at higher concentrations.
Additionally, it is preferable that the dried joint compound provides a similar surface porosity to the face paper of gypsum drywall in order to generate an even gloss or sheen across a finished wall after painting. If the joint compound is more porous than the face paper of the drywall, the finish coat of paint will be visibly different over the areas of the wall that have been treated with joint compound. Application of primers, drywall surfacers, or even skim coating the entire wall with a thin layer of joint compound are examples of methods that have been used to minimize the porosity differences sometimes encountered in drywall finishing. Differences in porosity and texture and their effect on paint finishes are most obvious when the wall assembly is finished by painting with a gloss or a semi-gloss paint. In order to produce a painted surface that looks uniform from various viewing angles, it is desirable to more closely match both the porosity and the texture of the joint compound with those properties at the paper surface of the gypsum drywall panel.
Joint compounds are also easier with which to work if certain properties are satisfied. Workability and slip of joint compounds make them easier to apply to the wall, easier to trowel to a smooth finish, and they dry less quickly, giving the novice applicator more time with which to work with the joint compound.
There is, therefore, a need in the art for an improved joint compound that has improved application properties and shows higher levels of gloss retention under a variety of lighting. There is also a need for a joint compound that minimizes or localizes generation of airborne dust.
These and other needs are met or exceeded by the present invention that features a joint compound that minimizes or localizes dust when the dried or set joint is sanded.
More specifically, the present invention provides a low dusting joint compound comprising a filler, a binder, a thickener and one or more synthetic polymeric waxes that are at least slightly soluble in water and are solid at room temperature. Preferably the wax is a high molecular weight polyethylene glycol or methoxy polyethylene glycol having an average melting point of from about 80xc2x0 F. (27xc2x0 F.) to about 150xc2x0 F. (80xc2x0 C.). This additive imparts low dusting properties without a sticky or oily feel. High molecular weight synthetic waxes also act as an internal binder to hold the joint compound together and improve the gloss retention of the joint compound.
Petroleum oils, residues and paraffins are known to reduce dusting in plasters or joint compounds however, they are all liquids by nature. Unexpectedly, the present inventors have found that high molecular weight synthetic waxes, in the form of solids, also have the ability to agglomerate particle fines during the cutting, abrading or sanding of joint compounds. The joint compound of this invention does not necessarily produce less dust or fines, but the fine particles of joint compound that are created during sanding are agglomerated into larger particles that are less likely to remain airborne. As used in this application, the concept of reducing dust refers to reducing of airborne dust or fines.
While not wishing to be bound by theory, it is believed that the hard waxes soften under the friction of abrasion, agglomerate the fine particles in the vicinity of the softened wax, and then harden again very quickly. Compared to liquid oils and waxes, the solid wax of this invention does not tend to migrate toward the surface of the workpiece resulting in an uneven distribution of the low dusting additive, and functions well as an internal binder, holding the joint compound together. The higher molecular weight particles also agglomerate the dust fines without any stickiness or oiliness. The agglomerated particles often fall cleanly away from the work surface, rather than sticking to the surface of the wall or tools, or becoming airborne for longer time periods and settling a farther distance away from the sanding area. These features result in a product that requires less extensive clean up compared to conventional joint compounds.
Another feature of this composition is the improved gloss retention when paint is applied over the surface of the joint compound. Perfect gloss retention would produce the same paint gloss level over the joint compound as the paint gloss level over the paper surface of the wallboard (100% gloss retention). Poor paint gloss over drywall seams treated with joint compound can result in walls in which the paint over all the dried joint compound has a lower sheen than the rest of the painted wall, creating an uneven appearance. Such xe2x80x9cbandingxe2x80x9d or xe2x80x9cflashingxe2x80x9d can require additional finishing steps, such as skim coating, or application of primers to achieve an acceptable result. The improved gloss retention for joint compounds made with synthetic polymeric wax results in a product that can produce a more uniform appearance to the final decorative painted surface, with less effort, than conventional joint compounds.